In recent years, secondary batteries applicable even in a steep load change in large capacity have been developed and the development of railroad vehicles equipped with such secondary batteries has also been progressed. Therefore, power peak-cutting technology in which the secondary battery (power storage device) absorbs a steep change in power consumption or regenerative power occurring during power running or braking of the vehicle and hence a voltage change due to a power change is prevented from occurring on an overhead wire has been studied. Thereby, it is possible to reduce the costs necessary to manufacture and maintain in-vehicle electric components or ground facilities.
In addition, as technology related to this, a method of constantly maintaining a power system having a power storage device in a normally operable state constantly by appropriately controlling a state of charge (SOC) of the power storage device based on a voltage change of the overhead wire has been disclosed (Patent Literature 1). Here, for charging and discharging of the power storage device, as disclosed in Patent Literature 1, a technique of mounting an overhead voltage detection device and a charging and discharging control device and controlling a charging/discharging amount of the power storage device depending on the overhead wire voltage has been adopted.
In particular, in the charging and discharging control device disclosed in Patent Literature 1, means in which a predetermined charging or discharging voltage threshold value is set and charging or discharging is performed in the power storage device when a detected overhead wire voltage is greater than or equal to or less than the predetermined charging or discharging voltage threshold value is taken. This is to prevent the life of the power storage device from being shortened due to free charging or discharging through a voltage change in the overhead wire. Accordingly, when the overhead wire voltage is greater than or equal to the above-described discharging voltage threshold value and less than the charging voltage threshold value, the charging and discharging control device is controlled so that the power storage device is not freely charged and discharged in a relation to the overhead wire voltage.